Email Migration API v2: Google Apps Script

I am trying to write a GAS to migrate some emails which are stored in Google Drive but I am really struggling to get the POST correct to do this and was hoping someone could help me out and steer me in the right direction.

What I have so far is ..

  var id = "12345678abcdefgh";
  var doc = DocumentApp.openById(id);
  var emlData = doc.getText();
  var api_scope = 'https://www.googleapis.com/auth/email.migration';
  var app_name = "migration";
  var userKey = "someone@mygappsdomain.com";
  var method = "POST";

  var url = "https://www.googleapis.com/upload/email/v2/users/"+userKey+"/mail?uploadType=multipart";


  var fields =  {"MailItem" : 
                  {"properties":
                   {'isInbox': 'true','isUnread': 'true'},
                   'labels': ['MigrateMe']}};

  var options = {payload: {data: JSON.stringify(emlData), fields: fields, contentType: 'multipart/related', boundary : 'part_boundary'}};
  var fetchArgs = googleOauth_(app_name,api_scope,method,options);
  try
  {
    var result = UrlFetchApp.fetch(url, fetchArgs).getResponseCode();
    Logger.log("done");
  }
  catch (ee) 
  {
    Logger.log(ee);
  }
}

This obviously doesn't work and I get a 400 error code. Do you know what could be wrong ?

As spoken in comment here an exemple to do it with the playground api:

step 1:
Authorise: https://www.googleapis.com/auth/email.migration

step 2:
Authorize blbablablabla (you know what to do here)

step 3:
You need to do a POST to hte URL:
https://www.googleapis.com/upload/email/v2/users/EMAILADRESS@DOMAIN.EXT/mail

The Content-type need to be set on: custom...
and the arguments are:

uploadType:media
Content-Type:message/rfc822

the request body must look at something like this:

Date: Wed, 03 Jan 2013 02:56:03 -0800
From: admin@example.org
To: liz@example.com
Subject: Hello World!
MIME-Version: 1.0
Content-Type: text/html; charset=windows-1252
Content-transfer-encoding: 8bit

And I think to myself... What a wonderful world!

(you can take exactly this one to do some test and then in your mailbox search for "liz")

If everything worked you should have something like this:

To do this I used the documentation found here:
https://developers.google.com/admin-sdk/email-migration/v2/guides/upload
https://developers.google.com/admin-sdk/email-migration/v2/reference/mail/insert

This is not the solution to your post, but I Hope it will help you to get it! (and by the way if you get to have a nice working code please give the real answer to your question - I'm also interested)